Chromel is an alloy composed of nickel. iron, and chromium. A 0.6459-g sample wa
ID: 524345 • Letter: C
Question
Chromel is an alloy composed of nickel. iron, and chromium. A 0.6459-g sample was dissolved and diluted to 250.0 mL. When a 50.00-mL aliquot of 0.05173 MEDTA was mixed with an equal volume of the diluted sample, all three ions were chelated, and a 5.33-mL back-titration with 0.06139 M copper(II) was required. The chromium in a second 50.0-mL aliquot was masked through the addition of hexamethylenetetramine; titration of the Fe and Ni required 38.89 mL of 0.05173 MEDTA. Iron and chromium were masked with pyrophosphate in a third 50.0-mL aliquot, and the nickel was titrated with 27.17 mL of the EDTA solution. Calculate the percentages of nickel, chromium, and iron in the alloy. Percentage of nickel = % Percentage of iron = % Percentage of chromium = %Explanation / Answer
50 ml aliquot analysis,
Total EDTA used for (Ni + Fe + Cr) = 0.05173 M x 50 ml = 2.5865 mmol
Total (Ni + Fe + Cr) moles present = 2.5865 - 0.06139 M x 5.33 ml = 2.2593 mmol
In 250 ml total (Ni + Fe + Cr) moles present = 2.2593 x 250/50 = 11.2965 mmol
50 ml aliquot analysis,
Cr masked, remains Ni + Fe
Total EDTA used = total Ni + Fe present = 0.05173 M x 38.89 ml = 2.0118 mmol
moles Cr present in original solution = 11.2965 - 250 x 2.0118/50 = 1.2375 mmol
Percent of chromium = 1.2375 x 10^-3 x 51.9961 x 100/0.6459 = 9.96%
50 ml aliquot analysis,
Fe and Cr masked, remains Ni
Total moles of Ni in aliquot = 0.05173 M x 27.17 ml = 1.4055 mmol
moles Ni in original solution = 1.4055 x 250/50 = 7.0275 mmol
Percent of nickel = 7.0275 x 10^-3 x 58.6934 x 100/0.6459 = 63.86%
moles Fe in original solution = 11.2965 - (7.0275 + 1.2375) = 3.0315 mmol
Percent of iron = 3.0315 x 10^-3 x 55.845 x 100/0.6459 = 26.21%
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